Owing to the increased significance of environmental protection, recent research and development work in the battery field has been concentrated on decreasing or eliminating the use of mercury in primary and secondary cells. A low mercury cell cannot comprise more than 0.025 by weight of mercury which is equivalent to about 0.25% by weight of mercury content in the anode. (Hereafter, the term "wt. %" is used to indicate a percentage by weight of the indicated component).
It can be expected that in the foreseeable future the commercial sale of mercury containing cells will be prohibited in most industrialized countries.
While the elimination of the mercury content in cells is a reasonable requirement in view of environmental protection, this causes numerous problems regarding cell operation and performance.
These problems have been summarized in detail in the paper of D. von Borstel entitled: "Das (uberwundene) Quecksilberproblem der Primarzellentechnik"--in English translation: "The (Overcome) Mercury Problem of Primary Cells Technique"--published in Dechema-Monographien Band 124 - VCH Verlagsgesellschaft 1991 pp. 375-388.
These problems are connected mainly with the following properties of cells without a sufficient amount of mercury:
increasaed generation of hydrogen gas due to zinc corrosion;
decreased loadability of the zinc; and
decreased electrical conductivity of the zinc particles.
While the cited paper deals mainly with primary cells, in case of secondary (rechargeable) cells these problems are more serious, since the electrochemical processes will be repeated in all cycles; and with increased cycle number the initially less significant magnitude of these phenomena become more apparent.
There are different approaches to the ways how these adverse effects can be overcome. In the commercially available low mercury and mercury free alkaline manganese dioxide-zinc primary and secondary cells, corrosion inhibitor materials are added to the anode gel which decrease hydrogen evolution from the zinc particles. The use of these inhibitor materials causes, however, further problems, i.e., they decrease conductivity and cell performance under high current drains.
The performance of such commercially available low mercury or mercury free cells is noticeably inferior compared to those including mercury; they leak at elevated temperature, and in secondary cells the cycle capacity sharply decreases with increasing cycle number and therefore they have smaller cumulative capacity values than conventional cells have.
The object of the present invention is to provide a low mercury or mercury free alkaline manganese dioxide-zinc cell in which the above problems become less significant or are eliminated.
The invention is based on the discovery that the approach of using corrosion inhibitors is insufficient for solving the problems connected with the lack of mercury, since with increased corrosion inhibition properties the adverse effects will also be higher. Therefore, there will be no acceptable compromise concerning cell performance.
In contrast to this approach, it has been recognized that the corrosion of high purity zinc is a slow process, the hydrogen gas development lies in the range of 1-5 microliter/day/gram, and the thus developed hydrogen can be recombined. The purity of the zinc has, however, an increased significance, since in solutions contaminated with heavy metal ions or particles or other contaminating materials present in the cathode material (like iron, copper, nickel, manganese dioxide, graphite etc.) the gas development can be several thousand times as high as in case of pure zinc.
In accordance with the inventive approach, it has been discovered that starch, preferably epichlorhydrin modified starch, should be used in the anode as a gelling agent, preferably in an amount between about 0.5 to 4 wt. %. The use of starch as gelling agent has already been known in the prior art; however, it was always combined with a substantial amount of other gelling agents like carboxy methyl cellulose (CMC) or CARBOPOL, whereby such anode gels obtained gas bubbles retention properties.
Gels made using epichlorhydrin modified starch do not retain hydrogen bubbles developed in the anode.
The gas release properties of the anode gel improve still further if the gel contains 0.5 to 3 wt % magnesium oxide.
The gelling properties of epichlorhydrin modified starch improve substantially if the anode gel is made at elevated temperatures between about 40.degree. C. and 65.degree. C., preferably between 50.degree. C. and 60.degree. C.
In accordance with the inventive concept, care should be taken that an inside pressure build up within the cell will not exceed permitted limits. Therefore, the cell comprises a hydrogen recombination system. Such systems are known in the art and, based on their operational principles, can be grouped in one of two types. The first type is the fuel cell electrode hydrogen recombination system which operates electrochemically, and the second type is a chemical recombination system using a hydrogen recombination catalyst. The fuel cell type hydrogen recombination systems use generally an auxiliary electrode as disclosed in U.S. Pat. No. 4,925,747 issued to Kordesch et al, and their operation does not influence the rechargeability of manganese dioxide. The second type reduces manganese dioxide to crystalline compounds which are not reversible, and the reduced manganese dioxide cannot be recharged any more. However, if the rate of gas evolution is low, the loss of rechargeability can be negligibly low; therefore, this type of hydrogen recombination system can also be used a suggested in the cited in U.S. Pat. No. 4,925,727.
It belongs to the inventive concept to utilize all means to slow down the pressure increase in the cell and to increase the cell performance.
It has been found that most of the materials that increase corrosion come from the porous manganese dioxide cathode in the form of dust, and this kind of contamination can be reduced efficiently if a protective coating is provided on the cathode surface--which coating is an ion permeable and ionically conductive layer that enables, normal cell operation. The thickness range of the coating can vary between about 25 and 200 microns and the coating can be made from a solution that comprises 2 to 5 wt. % starch, a viscosity control additive and 1-5 wt. % magnesium oxide. The presence of starch in the layer is preferable, since it has good soaking properties with the KOH electrolyte and contributes to preserving the required electrolyte content of the cathode.
A further corrosion source can be the metal current collector for the negative terminal that must be in contact with the anode gel and which is made generally of brass, copper or bronze. According to a further inventive aspect, the corrosion caused by the current collector on overdischarge can be eliminated or reduced to a negligible extent if the current collector is coated with gold.
In view of hydrogen evolution, it is preferable that a 9 to 12N potassium hydroxide forms the electrolyte, since the rate of zinc corrosion at such concentrations is lower than in commonly used lower concentration KOH electrolytes.
A further drawback of mercury free anode gels is their decreased interparticle conductivity. This drawback can be eliminated if the anode gel comprises at least 0.1 wt. % of particles with gold coated surface. Such particles can take any form, i.e. they can be fibers, flakes, etc. In the case of fibers, it is preferred that they have a length to diameter ratio between 100:1 and 1000:1 and that they are plated with gold.
The cells according to the invention can both be primary or secondary cells. The combination of the above summarized means enables the production of mercury free rechargeable cells with performance comparable to cells containing mercury.
Such cells can be of the commonly used cylindrical form, but any other cell configuration can be well within the scope of the invention.